1. Field of the Invention
The present invention relates to a developer for developing latent electrostatic images to visible images in a developing process in the fields of electrophotography, electrostatic recording and electrostatic printing; and a method of forming images by using the developer.
2. Discussion of Background
A variety of two-component type developers have been proposed, as disclosed in U.S. Pat. Nos. 2,618,551, 2,618,522, 2,573,881, and 2,638,416 since the invention of an electrophotographic method by C. F. Carlson (U.S. Pat. No. 2,297,691). In addition, after the invention of magnetic brush development (U.S. Pat. No. 2,786,439), many proposals have been made to improve the charging characteristics and moisture-resistance of a developer and prevent the scattering of toner particles in the image forming apparatus by coating a carrier with a resin.
The two-component type developer comprises a toner and a carrier. The toner is charged to a predetermined polarity and transported to a development zone by the action of the carrier, and only the toner is attached to an electrostatic latent image to develop it to a visible toner image.
For example, a non-coated carrier comprises magnetic particles such as iron particles, magnetite particles, and ferrite particles. This kind of magnetic powder carrier is simple in structure and excellent in durability. However, the toner cannot satisfactorily be charged by the magnetic powder carrier because the resistivity of the carrier is relatively small, thereby producing the problem of toner particles scattering in the image forming apparatus and the problem of fogging. Furthermore, due to a relatively heavy weight of the magnetic powder carrier, the toner is easily deposited on the carrier to induce the spent phenomenon while the toner and the carrier are mixed and stirred in a development unit. When the spent phenomenon occurs, the charge-imparting capability of the carrier deteriorates, which has an adverse effect on the obtained images, such as occurrence of fogging and decrease of image density. Consequently, the life span of the developer is reduced and regular replacement of the developer becomes inevitable.
A coated-type magnetic powder carrier prepared by coating the surfaces of the magnetic particles with a resin can prevent the scattering of toner particles in the image forming apparatus and the fogging of the obtained images. This is because the resistivity of the coated-type magnetic powder carrier is increased, and therefore, the charging characteristics of the toner can be improved. However, it is inevitable that the coated resin layer be peeled away from the magnetic particle during the repeated operations. In this case, the toner particles are scattered in the image forming apparatus, and the fogging phenomenon occurs. Particularly, the coated resin layer is easily peeled away to decrease the durability of the carrier in a small-sized image forming apparatus equipped with a compact development unit because a large shear force is applied to the carrier in such a small-sized development unit in the course of mixing and stirring of the carrier and the toner.
There are conventionally known a magnetic resin carrier with a small diameter, prepared by dispersing finely-divided magnetic particles in a binder resin, and a coated-type magnetic resin carrier prepared by coating the above-mentioned magnetic resin particles with a resin, as disclosed in Japanese Laid-Open Patent Applications 47-13954, 49-51950 and 50-2543. Owing to the presence of a resin, not only the resistivity of the magnetic resin carriers is increased to improve the capability of imparting the charge to the toner, but also it is possible to weaken the stress applied to the carrier to lengthen the life of the obtained developer, as reported in "Plain Paper Copier for High Image Quality Using New Process and New Developing Materials" (National Technical Report, Vol. 26, No. 4, Aug., (1982)). However, since the specific gravity, that is, the true density of the magnetic resin carrier is small, the carrier is easily attracted to the surface of an electrophotographic photoconductor and deposited on latent electrostatic images thereon together with the toner in the course of the development process.
Furthermore, a developer disclosed in Japanese Laid-Open Patent Application 59-192262 comprises an electrically insulating toner, and a mixture of a magnetic resin carrier comprising a binder resin and finely-divided magnetic particles dispersed in the binder resin and a magnetic carrier comprising ferromagnetic powder. This developer forms a magnetic brush with flexible fibers, and avoids the coagulation of the magnetic carrier particles. However, the toner for use in this developer is a conventional electrically-insulating toner, so that the spent phenomenon occurs during the continuous printing operation, thereby degrading the quality of the obtained images.
The addition of a third material to the toner and the carrier is proposed to prevent the deterioration of the carrier, as reported in a paper entitled "PPF Method and How to Improve the Quality of Copied Images" by Makoto Sumida in the Journal of the Institute of Electrophotography Engineers of Japan, Vol. 23, No. 1, (1984). In this case, it is difficult to maintain the image density constant because the above-mentioned third material is easily attracted to the photoconductor together with the toner.
In the development system in which the toner clings to the carrier by electrostatic attraction, the electrostatic adhesion between the toner and carrier varies depending on the environmental change. Particularly, the toner is easily scattered in the image forming apparatus in the atmosphere of high humidities. The reason for the decrease in electrostatic adhesion between the toner and the carrier is considered that the spent toner is attached to the surface of the carrier, and therefore, some toner particles are insufficiently charged and others are charged to a polarity opposite to the predetermined one.
With respect to a photoconductor, attention has recently been paid to an a-silicon (amorphous silicon) based photoconductor in addition to the conventional Se-based photoconductor and an organic semiconductor. The a-silicon (hereinafter referred to as a-Si) based photoconductor is superior to the Se-based photoconductor in terms with the safety and durability. The life of the a-Si based photoconductor is longer than that of the image forming apparatus itself.
However, the a-Si based photoconductor is apt to induce the so-called image blurring because the electric charge readily leaks from the photoconductor when the operation is extended over a long period of time.
To solve the blurring problem, an abrasive substance is conventionally utilized. More specifically, the addition of finely-divided particles of strontium titanate to a developer is proposed, as disclosed in Japanese Laid-Open Patent Application 61-278861; a cleaning member with a Mohs hardness of 2.5 to 7.0 is used, as in Japanese Laid-Open Patent Application 59-88776; and finely-divided abrasive particles with almost the same Mohs hardness as that of a surface layer of the a-Si based photoconductive drum is contained in a developer as in Japanese Laid-Open Patent Application 63-29759. Further, as reported in Japanese Laid-Open Patent Application 61-231564, the addition of an alkaline earth metal and carbonate to the developer is effective for preventing the blurring from happening on the a-Si based photoconductor.
However, the abrasive effect obtained by any of the above-mentioned methods with respect to the surface of the a-Si based photoconductor is insufficient to prevent the blurring on the photoconductor. To obtain a desired abrasive effect, the image forming apparatus necessarily becomes large in size, and the aforementioned abrading methods cannot be applied to a small-sized image forming apparatus.
Japanese Laid-Open Patent Application 59-192262 discloses a developer for use in electrophotography, which comprises an electrically insulating toner, a magnetic carrier comprising magnetic particles, and a magnetic resin carrier comprising a binder resin and finely-divided magnetic particles dispersed in the binder resin. The above application does not make any suggestion about the combination of the abrasive-type toner component and the carrier component.
The fixing of ceramic particles to the developer and the use of such a developer are described in Japanese Laid-Open Patent Applications 63-85756, 63-103264, 1-196072, 3-203743 and 3-43747.
The surface-modification of a basic particle by fixing finely-divided particles on the basic particle is described in Japanese Patent Publications 3-2009, 3-76177 and 4-3250, Japanese Laid-Open Patent Applications 62-262737 and 62-298443 and Japanese Utility Model Publication 4-45538.